The present invention relates to device for manipulating charged particles using an electric field. The preferred embodiment relates to a device for use in a mass spectrometer for manipulating ions.
It is desirable to use electric fields to manipulate ions in mass spectrometers. Typically, the device for manipulating the ions comprises a series of electrodes spaced apart along a longitudinal axis of the device. Voltages are applied to the electrodes in order to form the desired electrical potential profile along the device so as to manipulate the ions in the desired manner. The adjacent electrodes in these devices tend to be electrically connected to each other by resistors or capacitors in order to maintain each electrode at the desired potential. It may be necessary to use a number of resistors having different resistances or a number of capacitors having different capacitances in order to achieve the desired potential profile along the device. This complicates the manufacture of the device, particularly where different capacitors are required, as it is difficult to accurately alter the capacitance of a capacitor to a desired value.
An example of a device for manipulating ions in a mass spectrometer is an orthogonal acceleration Time of Flight (TOF) mass analyser. This typically comprises a series of regions of constant electric field which differ in electric field strength, such as acceleration regions and reflectrons. In order to support these fields in the bulk of the device where the ions fly, different voltages are applied to a series of discrete electrodes that closely mimic the boundary conditions of the desired internal or bulk electric field. In the example of a single stage reflectron, the reflectron is formed from a series of cylindrical electrodes of the same length that are arranged adjacent to one another and that are connected via a potential divider consisting of resistors of equal value. The resulting electric field has discontinuities close to the surfaces of the electrodes, but these discontinuities quickly relax away from the surfaces of the electrodes to provide a smooth, constant electric field that is desired for the operation of the analyser. It is desired to minimise the complexity and number of such electrodes, but to still obtain sufficient relaxation of the electric fields in the bulk of the device so as to allow successful operation of the device.
More complex, higher order electric fields may also be created along a device by applying the appropriate potential function to a series of electrodes spaced along the device. Provided that the desired bulk field is a supported field, i.e. it satisfies Laplace's equation, then the prudent application of a potential function to the discrete electrodes that closely follows the boundary condition along a defined geometrical surface will allow the electric field to quickly relax to the desired form. The accuracy of the bulk field will depend on the accuracy of the location of the electrodes and the voltages applied to them.
Although the desired potential profile may be achieved relatively easily for certain potential profiles, this becomes more difficult when it is desired for the potential profile to follow higher order functions. Problems are also encountered if the potential profile is required to be pulsed on an off. Electrodes that define a region which requires a pulsed electric field must have capacitive dividers between the electrodes so as to provide the different voltages to the different electrodes. However such dividers are generally of low tolerance and it is difficult to accurately provide the required capacitance for each capacitor. By way of example, such problems might occur in the pulsed ion extraction region of an TOF mass analyser. It is desired to provide an improved method of manufacturing a device for manipulating charged particles, an improved device, an improved mass spectrometer and an improved method of mass spectrometry.